The combustion of bark and other wood waste, flour fibres, lignite and other fuels burnt with difficulty is only really successful with the aid of a fluidised layer combustion device. This fuel has traditionally been either located separately beside the boiler proper from which hot combustion gases are led to heat up the boiler vaporiser and convection surfaces, or as a permanently constructed fixed unit in the bottom part of the boiler. In soda boilers in the vicinity of a cellulose factory use of the latter mentioned fluidised combustion device in this manner has not been possible since conversion of the boiler thereafter as a soda boiler could not be achieved. It has only been possible to use fluidised layer combustion devices when located beside a separate boiler.
The present invention eliminates the abovementioned drawback. In the present device the fluidised bed combustion device is built inside the soda boiler. The fluidising nozzles are detachably mounted on the air beams resting on the existing boiler base pipes. They can be, if required, rapidly removed and installed in place through the existing air feed openings in the side of the boiler firebox. These openings are blocked up by plastering in a closing door when the boiler is operating as a soda boiler.